What is the actual happening during solidification of friction stir welding of aluminum alloys that causes the formation of equiaxed grains in the nugget zone irrespective of process parameters. That is how it is different from the case where we obtain columnar or other shapes of grains. How dynamic recrystallization plays the role here.
First of all FSW is a solid state welding process. Therefore, no solidification is involved here.
Aluminum, having a high stacking fault energy (SFE), is easily amenable to dynamic recovery. High SFE results in easy cross slip and climb. The rate of recovery increases with dislocation density where dislocations re-structure easily to form low angle grain boundaries and subgrain structures that lead to formation of equiaxed grains. Strain rate, strain, and temperature are important, according to Zener-Holloman parameter. Therefore, process parameters play a role in dynamic recrystallization.
Thank you for your valuable answer to my query Krishnan Raja sir. Actually I have a specific question from a reviewer where he asked me about the reason behind the formation of equiaxed grains in nugget irrespective of tool pin design and process parameter. I have used 4 tool pin geometries and two process parameter combinations. Total 8 samples and all are showing equiaxed grains with varying diameter. The conditions are 1) 900 RPM and 2.25 mm/sec 2) 1500 RPM and 0.75 mm/sec. These combinations are used grossly to have a comparatively lower and higher heat input condition. My material is AA 6061 T6.
Can I say the formation of equiaxed grains in nugget is an indication of the happening of dynamic recrystallization?
I think your reasoning based on dynamic recrystallization will be acceptable to the reviewer. The varying grain size indicates that process parameter affect the kinetics of recrystallization. Your process parameters not only affect the frictional heat input but also the strain rate and strain that are important for formation and annihilation of dislocations.
I cannot throw more light on FSW of AA specific to your condition because I have not done any work on those. Only recently we started working on FSW of SS.
Dear Mr. Abhishek
I agree with the explanation given by Mr. Krishna.
Prof. A,V,Sethuraman
Hi,
It is well known that CDRX or GDRX is the main DRX mechanism during FSW/P of your used alloy system. Please note that the grain growth or grain boundary mobility is also a function of heat input or temperature during FSW.
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